The present invention relates to the molding of thermoplastic lens blanks and the like and, more particularly, to a method and apparatus for molding one or more lens blanks of the same or different optical power from a polycarbonate resin so as to form lenses having a minimum of stress.
In the past, ophthalmic lenses, instrument lenses and small windows have been molded from thermoplastic material such as polycarbonate resin because products so formed are light weight, relatively inexpensive and virtually indestructible. Molded lenses present special problems, however, because in order to be commercially acceptable, lenses must be free of blemishes and distortions and have edge portions free from starring which can result in nonuniform setting of the thermoplastic material within a lens mold cavity.
My earlier U.S. patent application Ser. No. 816,197, filed Jan. 6, 1986, now U.S. Pat. No. 4,664,854 the disclosure of which is hereby incorporated by reference, is directed to a method and apparatus for producing a thermoplastic lens having a minimum of stress. More particularly, the molding apparatus disclosed therein includes a male mold and a female mold which define therebetween a molding space. The molds are mounted for movement toward and away from each other, and a gate is mounted adjacent to one of the molds for the passage of thermoplastic resin. This mold defines a window width for admitting thermoplastic resin to the molding space and floats away from the other mold in response to the admission of thermoplastic resin. A compression element operatively engages the floating mold and is movable from a home position to an end position so as to move the mold towards the other mold and to compress thermoplastic resin within the molding space. Finally, the home position of the compression element is adjustable to adjust the initial window width and, hence, balance thermoplastic resin fill speed and control the compression pressure applied to the thermoplastic resin within the molding space. In the preferred embodiment of this apparatus, the compression element is a wedge-shaped slide member and the gate includes a retainer ring member which surrounds the floating mold. A schematic representation of a molding apparatus of this type is shown in FIG. 1.
The present invention is directed to an improvement in the molding apparatus disclosed in my prior application. More particularly, it has been found that after the mold is repeatedly used, dust particles and other foreign substances can accumulate between the retainer ring and the free floating mold. This increases the friction between these elements so that the free floating mold does not move rearwardly as easily or as quickly when resin is injected. To overcome such initial inertia, the injection pressure of the thermoplastic resin of my prior apparatus must be increased. However, increasing injection pressure is undesirable for it can lead to an increase in stress in the lens formed at the molding station. Accordingly, the object of the present invention is to enable the "floating" or moving mold to be moved rearwardly automatically prior to the injection of thermoplastic resin. Thus, initial inertia need not be overcome and a smaller pressure of injection may be utilized.
Further, in a molding apparatus formed in accordance with my prior application, the retaining ring surrounding the floating mold must have a shape corresponding to the shape of the stationary mold because the stationary mold is larger than the floating mold. Accordingly, when the stationary mold is changed so as to provide a different curvature, it is also necessary to change the retainer ring.
In addition, it has been found that the fixed mold, the male mold in the preferred embodiment, needs to be changed more often than the female mold when forming various lenses. The need to repeatedly change the male mold necessitates the provision of a plurality of retainer rings of different curvatures.
Accordingly, it would be desirable to provide a retainer ring that need not be changed each time the curvature of the fixed mold is changed. A further desirable characteristic is a thermoplastic gate that is as large as possible so that the injected resin may flow as freely as possible to the molding space, limited only by the window width. Further, when higher powered lenses, i.e., thicker lenses, are formed with my earlier apparatus, the resultant edge angles are very small and may lead to high stress at the very edges. Thus, an apparatus which can make smaller diameter lenses with the same power thickness, will avoid undesirable stress at the edges.
Yet a further desirable feature is the provision of a device for independently controlling the speed for compacting and withdrawal of each of the hydraulic cylinders. With such control, each compression element will reach its home position and end position at the same time. Thus, compression can take place later for thicker lenses and pressure can be better controlled.
By way of improving my earlier invention, the present invention provides a molding apparatus which includes a male mold and a female mold defining therebetween a molding space. The molds are mounted for movement towards and away from each other. Further, a gate is mounted adjacent to at least one of the molds for passage of thermoplastic resin, that mold defining a window width with the gate for admitting thermoplastic resin to the molding space. A compression element is also provided for operatively engaging one of the molds and is movable from a home position to an end position so as to move the mold toward the other mold to compress thermoplastic resin within the molding space. In accordance with the present invention, the window defining mold is moved automatically to a maximum distance from the other mold prior to the injection of thermoplastic resin.
As in my prior invention the home position of the compression element is adjustable to adjust the window width and, hence, balance thermoplastic resin fill speed and control the compression pressure applied to thermoplastic resin within the molding space.
In the preferred embodiment of the present invention, the gate is formed by a retainer ring member which surrounds the movable, window defining mold and a retainer ring member which surrounds the other, stationary mold. Thus, the retainer ring surrounding the movable mold need not have a curvature matching the curvature of the stationary mold and, accordingly, the latter can be changed without necessitating changing the retainer ring. Further, the gate provided can be larger and thicker lenses may be formed without too sharp an edge angle.
Finally, in another preferred embodiment of the present invention, a valve controlled, for example, by a computer is provided in a line to each hydraulic cylinder so that compression can take place later for thicker lenses and all the compression elements or wedges reach their home positions and end positions at the same time. Thus, pressure can be controlled more precisely.